How did OD&D mechanically define characters? Sidebar: I don't care about roleplaying differentiation. Obviously you can play one fighter as a noble knight errant and another as a thieving brigand. I am referring to the system's actual crunchy differentiation.

Class: Fighter / Magic-User / Cleric. This differentiation defined a lot of your mechanics used, abilities, and so on. I would argue that the most important thing determining what class a player should pick is their experience. Novice players would have the most success with fighters, which were relatively simple and easy to play. High level fighters had access to magic swords which made them a little more interesting. Middling players could pick clerics, who started to introduce some special abilities like turn undead and spells, although a level 1 cleric had no spells. Finally, advanced players might select a magic-user, which was definitely a challenge to keep alive but theoretically had the most power if played wisely. A party didn't necessarily need all three roles filled but it helps a lot. Dwarves are basically a subclass of fighters, and elves are the "fifth wheel" jack of all trades, switching as needed between fighters and magic-users.

Primary Ability Score: Your best ability score largely drove what class you should pick. You'd get an XP bonus and sometimes other minor bonuses based on this score. Sure, you could play a fighter with 17 INT and 10 STR but it probably isn't smart. This basically allocates roles randomly if you use random ability score generation. This is good with an experienced crew because it ensures all the key roles are filled, but it is bad because the roles may not be allocated according to player skill.

Other Ability Scores: Other abilities had minor effects which served to differentiate characters of hte same class. A fighter with 18 DEX might play a little differently than a fighter with 18 CHA.

Spells or Equipment: Magic Users could pick different spells which made them different from any other MU; martial types could use different weapons or armor.

AD&D expanded on this somewhat, but not fundamentally. You basically had four core roles (Ftr/MU/Cleric/Thief) which were still based on player skill. I'd argue that fighters and then clerics are the easiest to pick up, and thieves and mages the hardest to play well. Ability score generation might allow you to arrange the scores to taste which gave the player more control over the character generated; a mixed blessing because you might end up stuck in one role ("Well, your cleric just died... But we really need a cleric on the team... So why don't you drop that 17 in WIS and make another one?"). More subclasses and more important ability score riders basically improved differentiation between the roles, as well as some cross over. A paladin and fighter both filled the same role but the paladin had a bit of cleric flavor too. Race also became a seperate thing; this doesn't seem to have a systematic effect on the system, however. Multi and dual classing allowed a bit more mixing between previously rigid niches.

4E is the most radical departure in some ways. In 4E, characters are broken out as follows:

Role: Defender, Leader, Striker/Controller (DPS). This is the key thing that a party needs one of each to be balanced. There is also some relationship to ability scores. For example, leaders generally don't use DEX (and often prefer CHA).

Power Source: Martial, Arcane, Divine, Primal, Etc. This determines the flavor that a character uses to accomplish their role's responsibilities. It also has a loose relationship to ability scores. For example, many arcane characters favor INT (but not all). This actually encourages a party to all pick different power sources so as to cover all the different ability scores.

Race: Races sometimes have a slight leaning towards a role. For example, Half-Elves feel like leaders. However, there is no strong niche protection. Every party doesn't need an elf; every party does need a defender though.

Class: A combination of Role + Power Source. For example, a Divine Leader is a cleric. A Martial Leader is a warlord. Sometimes there are two options within a given combination; rogues and rangers are both Martial Strikers. Thus, class provides some degree of differentiation, almost like subclasses did in AD&D.

Primary Ability Scores: Each character usually has two primary ability scores that they are pumping throughout their career. One will be mandatory and determined by their class. Most classes then offer two other options to pick from which will have your character "lean" towards another role. For example, all shamans will be wise leaders; you can pick INT to lean towards striker though or CON to lean towards defender.

Other Ability Scores: These provide minor differentiation.

Feats: These provide differentiation between characters so that no two Rangers play the exact same.

Powers: Similar to spell and equipment selection in previous edition, these differentiate between characters so that no two Rangers play the exact same.

Noonan writes, "the roles came from somewhere, but they didn't inevitably emerge from player psychology. Players are all too happy to hop roles whenever they get the chance." That is true, in some cases.

In older versions of D&D, I think the most succesful role hopping was in order as player skill improved: Fighter --> Cleric/Thief --> Magic-User. Assuming a group of equally skilled players, however, players might hop roles whenever they got the chance. AD&D encourages this somewhat by making it relatively easy to replace fighters with henchmen, hirelings, and even summoned creatures. There are also several more complicated subclasses for the meatshield role (Defenders/Rangers; more than any other set of sub-class!). That way a group of very experienced players could basically leave out plain fighters in order to play more complicated (and to them, interesting) characters. AD&D also allowed players to arrange ability scores to taste in many cases, which gave players more control over what they played.

In 4E, role hopping isn't really based on player skill. You do need to fill all four roles (although some argue that controller/striker are redundant), though. While there is some preference, I do think that most players are happy to hop roles when they get the chance.

Where does this get us? Well, I've got some thoughts on how to bring this together in a more coherent manner.

ROLE: Roles basically serve as niche protection. They are specialties that your character is the best at. Some examples might be Defender/DPS/Healer, or Pilot/Engineer/Gunner. A game that has niche protection needs at least two different niches. However, if each niche is vitally important (and they should be; who wants to play a role that doesn't matter?) then the more niches you have, the larger the player group must be. For example, 4E is predicated on a group of five players; thus there are four roles to fill. I personally think that a group size of 3-4 players is most reasonable and attainable, especially these days, so something like three niche roles is probably good.

I think some games have a problem with too many niches. For example, Shadowrun has Street Samurai, Adepts, Faces, Hackers/Deckers, Riggers, and Magicians. You'd need six players to cover each role. Plus, the roles are so highly specialized and use unique rule space that I think it makes it hard to have a coherent and interesting adventure. The rigger -- specialized in operating vehicles -- will love a car chase but the adept will probably suck at it.

You can have more niches with smaller groups by using some contrivances:

Make some niches easily replaceable. AD&D does this with fighters. It is easy to find meatshields.

Make it easier to cover multiple niches. In AD&D, multi and dual classing allows some covering of multiple niches with one character. Interestingly, the dual-classing mechanics make it very easy to cover the fighter role (hrm... poor fighter...). Many of the best dual-classes start as fighters or clerics then switch to something else.

Have sub-classes that lean towards a second niche. AD&D has a paladin (fighter leaning cleric) and ranger (argueably a fighter leaning thief). 4E explicitly has many classes lean towards different sub roles. Paladins (defenders) can lean striker or leader, for example. If you're missing a key niche, then having two characters minor in it can help cover. For example, if an AD&D party is missing a pure cleric, then you could cover for it by having a paladin and a multiclass cleric around.

MEANS: The means that a character uses are how they fill their role. This, I argue, should be based on player typology. I think you could either use the Bartle CARD roles or the WOTC player roles study or Myers Briggs types. In theory, a given player will probably have a preference for one means over another -- and that's ok. In theory, players who are interested in social interaction with their other players should be rewarded by the system for that. Allowing some bleed over between roles isn't a bad idea as not every player can be easily pigeon holed.

PLAYER SKILL LEVEL: You should be able to set gamplay to easy for a new guy or hard for an experienced veteran. I think everyone's seen a novice player struggle with playing a magic-user at the AD&D table and get frustrated with the experience; they just want to sling spells and they keep dying and getting expected to carefully ration resources! Ideally, I think there'd by a way to move from an easy set up to a harder set up throughout play. THat was, as a player gains experience, they can dial up the difficulty without making a new character. These are some ways to control difficulty of play:

Nova: Easy means that the player has little control over how quickly their offensive resources are used up. Fighters have a good THAC0 chart and good AC all the time, but find it hard to dish out a ton of hurt in a short period of time. A harder play style requires the player to carefully dish out resources at critical moments. A first level magic-user only gets one Sleep bomb. It is like a nuke, but the player must carefully decide when to drop it.

Choice: Easy means fewer choices. Fighters choose what armor to wear and what weapon to wield and that's about it. Moreover, in OD&D, weapon choice doesn't even really matter that much. AD&D introduces some bad choices (fighters shouldn't specialize in clubs) but fighters still have few options. Hard means more choices. Spell casters must carefully assemble their spell books and choose what to memorize each day.

Mobility: Easy characters are not very mobile. They don't derive much benefit from positioning (i.e. it doesn't matter where they are) and can't get around very easily. Hard characters derive a lot of benefits from being in the right place in the right time and have the mobility needed to make it happen.

Mechanics: Easy characters use one unified mechanic. Roll high on a D20. Hard characters may require players to do more complicated dice gymnastics.

POWER SOURCE: How does the character accomplish their role? What are the "special effects" associated with their actions? Some examples might be arcane, divine, martial, etc. In a sci-fi game, maybe it is Hacking, Cybernetics, Psionics, or Vehicles. I think this is more campaign dependent than anything else. You could overlap this with one of the other factors above as desired. For example, AD&D overlaps power source with Player Skill (Arcane = Hard, Divine = Medium, Martial = Easy). I think it would also be cool to allow players to change power sources in play. For example, if you want to make magic rare and special, it could be something that can be gained only later in a character's career after a special quest rather than from level 1 on up. As another example, perhaps Primal power can only be obtained after going on a Spirit or Vision Quest.

DIFFERENTIATION: How are otherwise identical characters differentiated? Are there little unique traits associated with ability scores, feats, or other choices?

That gives you 36 different combinations before factoring in power sources or differentiation, or any options to "hybrid" or multiclass. For example, if you have no feats/ability scores for differentiation, and only one power source, then you can have 36 combinations. You might have a Thinker Easy Tank, or an Explorer Hard DPS.